Laser driver circuit and system

ABSTRACT

A method, system, and electrical circuit for determining a magnitude of a modulation current provided to a laser device based upon an approximated slope efficiency, wherein the approximated slope efficiency is based upon at least one discrete incremental change in a bias current to the laser device and at least one change in output power from the laser device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/321,177 filed on Dec. 16, 2002, now U.S. Pat. No. 6,928,094, theteachings of which are herein incorporated by reference.

BACKGROUND

1. Field

The subject matter disclosed herein relates to data communicationsystems. In particular, the subject matter disclosed herein relates totransmitting data in an optical transmission medium.

2. Information

Data transmission in an optical transmission medium such as fiber opticcabling has enabled communication at data rates of 10 gigabits persecond and beyond according to data transmission standards set forth inIEEE Std. 802.3ae-2002, Synchronous Optical Network/Synchronous DigitalHierarchy (SONET) protocol as indicated in a set of standards providedby the American National Standards Institute (ANSI T1.105.xx) orSynchronous Digital Hierarchy (SDH) as indicated in a set ofrecommendations provided by the International Telecommunications Union(e.g., ITU-T G.707, G.708, G.709, G.783 and G.784). To transmit data inthe optical transmission medium, a laser device typically modulates anoptical signal in response to a data signal.

FIG. 1 shows a schematic diagram of a prior art laser driver circuit 2to provide power to a laser diode 6. In response to a pulse data signal4, the laser driver circuit 2 provides a pulse current signal 12 and anominally fixed bias current (not shown) to the laser diode 6. Inresponse to the pulse current signal 12, the laser diode 6 transmits alight signal 10 having an output power 14. A photodiode 8 measures theoutput power 14 to be used in evaluating the performance of the laserdriver circuit 2 or the laser diode 6.

A “slope efficiency” typically expresses an efficiency of a laser devicein generating an output power in response to an input current signal.For example, a slope efficiency is typically expressed as a measurementof a change in output power of a light signal from a laser devicedivided by a magnitude of a change in input current signal provided tothe laser device to transmit the light signal when the laser device isproperly biased. The slope efficiency associated with a particular laserdevice typically changes as a function of age or operating temperature.For example, FIG. 2 shows a graph illustrating effects of temperature(i.e., different temperatures T₁, T₂ and T₃ on a slope efficiency of alaser device. In the illustrated example, the laser device has a higherslope efficiency at lower operating temperatures.

BRIEF DESCRIPTION OF THE FIGURES

Non-limiting and non-exhaustive embodiments of the present inventionwill be described with reference to the following figures, wherein likereference numerals refer to like parts throughout the various figuresunless otherwise specified.

FIG. 1 shows a schematic diagram of a prior art laser driver circuit toprovide power to a laser device;

FIG. 2 shows a graph illustrating typical effects of temperature on aslope efficiency associated with a laser device;

FIG. 3 shows schematic diagram of a system to transmit in and receivedata from an optical transmission medium according to an embodiment ofthe present invention;

FIG. 4 shows a schematic diagram of physical medium attachment andphysical medium dependent sections of a data transmission systemaccording to an embodiment of the system shown in FIG. 2;

FIG. 5 shows a schematic diagram of a laser driver circuit according toan embodiment of the physical medium dependent section shown in FIG. 4;

FIG. 6 shows a flow diagram illustrating a process of adjusting anoutput current of a laser driver circuit according to an embodiment ofthe laser driver circuit shown in FIG. 5;

FIG. 7 shows a graph illustrating changes of an output power of a lightsignal from a laser device in response to changes in an output currentfrom a laser device according to an embodiment of the process shown inFIG. 6;

FIG. 8 shows a laser driver circuit according to an embodiment of thelaser driver circuit shown in FIG. 5; and

FIG. 9 shows a flow diagram illustrating a process according to anembodiment of the process shown in FIG. 6 and the laser driver circuitshown in FIG. 8.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrase “in one embodiment” or “an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in one or moreembodiments.

“Machine-readable” instructions as referred to herein relates toexpressions which may be understood by one or more machines forperforming one or more logical operations. For example, machine-readableinstructions may comprise instructions which are interpretable by aprocessor compiler for executing one or more operations on one or moredata objects. However, this is merely an example of machine-readableinstructions and embodiments of the present invention are not limited inthis respect.

“Storage medium” as referred to herein relates to media capable ofmaintaining expressions which are perceivable by one or more machines.For example, a storage medium may comprise one or more storage devicesfor storing machine-readable instructions or data. Such storage devicesmay comprise storage media such as, for example, optical, magnetic orsemiconductor storage media. However, this is merely an example of astorage medium and embodiments of the present invention are not limitedin this respect.

“Logic” as referred to herein relates to structure for performing one ormore logical operations. For example, logic may comprise circuitry whichprovides one or more output signals based upon one or more inputsignals. Such circuitry may comprise a finite state machine whichreceives a digital input and provides a digital output, or circuitrywhich provides one or more analog output signals in response to one ormore analog input signals. Such circuitry may be provided in anapplication specific integrated circuit (ASIC) or field programmablegate array (FPGA). Also, logic may comprise machine-readableinstructions stored in a storage medium in combination with processingcircuitry to execute such machine-readable instructions. However, theseare merely examples of structures which may provide logic andembodiments of the present invention are not limited in this respect.

A “data bus” as referred to herein relates to circuitry for transmittingdata between devices. A “multiplexed data bus” as referred to hereinrelates to a data bus that is capable of transmitting data among two ormore devices coupled to the multiplexed data bus. A multiplexed data busmay transmit data messages to a device coupled to the multiplexed databus according to an address associated with the device or a position onthe multiplexed data bus where the device is coupled. However, this ismerely an example of a multiplexed data bus and embodiments of thepresent invention are not limited in this respect.

An “optical transmission medium” as referred to herein relates to atransmission medium capable of transmitting light energy in an opticalsignal which is modulated by a data signal such that the data signal isrecoverable by demodulating the optical signal. For example, an opticaltransmission medium may comprise fiber optic cabling coupled between atransmitting point and a receiving point However, this is merely anexample of an optical transmission medium and embodiments of the presentinvention are not limited in this respect.

A “laser device” as referred to herein relates to a device to transmit alight signal in response to a power source. For example, a laser devicemay transmit a light signal in an optical transmission medium which ismodulated by a data signal. However, this is merely an example of alaser device and embodiments of the present invention are not limited inthese respects.

A “laser driver circuit” as referred to herein relates to a circuit toprovide power to a laser device to be used for transmitting a lightsignal in an optical transmission medium. For example, a laser drivercircuit may provide a controlled current signal to provide power fortransmitting the light signal. However, this is merely an example of alaser driver circuit and embodiments of the present invention are notlimited in these respects.

A laser driver circuit may provide a current signal to a laser devicehaving a “bias current” component combined with a data current componentwhich is modulated by a data signal. The data current signal may begenerated by modulating a “modulation current” with the data signal. Themodulation current may determine an extent to which the magnitude of thecurrent signal may deviate from the bias current component. However,this is merely an example of a bias current and modulation current, andembodiments of the present invention are not limited in these respects.

The strength of a light signal from a laser device may be associatedwith a measurable “output power.” For example, an output power from alaser device may be measured from a sensor such as a photodiode which isexposed to the light signal. However, this is merely an example of anoutput power associated with a laser device transmitting a light signaland embodiments of the present invention are not limited in thisrespect.

An “average output power” as referred to herein relates to anapproximation of the mean output power of a laser device over a timeperiod. For example, an average output power may be determined basedupon an integration of an output from a sensor over a period of exposureto a light signal generated by the laser device and subsequentnormalization. A “swing output power” as referred to herein relates toan amount by which an output power of a laser device may deviate fromits lowest value to its highest value over a time period. However, theseare merely examples of an average output power and swing output power,and embodiments of the present invention are not limited in theserespects.

A “slope efficiency” as referred to herein relates to a relationshipbetween a current signal provided to a laser device and a resultingoutput power of a light signal generated by the laser device in responseto the current signal. For example, a slope efficiency may be expressedas a change in output power divided by a magnitude of a change incurrent signal. However, this is merely an example of a slope efficiencyand embodiments of the present invention are not limited in theserespects.

Briefly, an embodiment of the present invention relates to a laserdriver circuit to provide a current signal to power a laser device. Abias current provided to the laser device may be changed while changesin the output power of a light signal from the laser device ismonitored. A slope efficiency associated with the laser device may bedetermined based upon the changes in the bias current and changes in theoutput power. However, this is merely an example embodiment and otherembodiments of the present invention are not limited in these respects.

FIG. 3 shows a schematic diagram of a system to transmit in and receivedata from an optical transmission medium according to an embodiment ofthe present invention. AD optical transceiver 102 may transmit orreceive optical signals 110 or 112 in an optical transmission mediumsuch as fiber optic cabling. The optical transceiver 102 may modulatethe transmitted signal 110 or demodulate the received signal 112according to any optical data transmission format such as, for example,wave division multiplexing wavelength division multiplexing (WDM) ormulti-amplitude signaling (MAS). For example, a transmitter portion (notshown) of the optical transceiver 102 may employ WDM for transmittingmultiple “lanes” of data in the optical transmission medium.

A physical medium dependent (PMD) section 104 may provide circuitry,such as a transimpedance amplifier (TIA) (not shown) and/or limitingamplifier (LIA) (not shown), to receive and condition an electricalsignal from the optical transceiver 102 in response to the receivedoptical signal 112. The PMD section 104 may also provide to a laserdevice (not shown) in the optical transceiver 102 power from a laserdriver circuit (not shown) for transmitting an optical signal. Aphysical medium attachment (PMA) section 106 may include clock and datarecovery circuitry (not shown) and de-multiplexing circuitry (not shown)to recover data from a conditioned signal received from the PMD section104. The PMA section 106 may also comprise multiplexing circuitry (notshown) for transmitting data to the PMD section 104 in data lanes, and aserializer/deserializer (Serdes) for serializing a parallel data signalfrom a layer 2 section 108 and providing a parallel data signal to thelayer 2 section 108 based upon a serial data signal provided by theclock and data recovery circuitry.

According to an embodiment, the layer 2 section 108 may comprise a mediaaccess control (MAC) device coupled to the PMA section 106 at a mediaindependent interface (MII) as defined IEEE Std. 802.3ae-2002, clause46. In other embodiments, the layer 2 section 108 may comprise forwarderror correction logic and a framer to transmit and receive dataaccording to a version of the Synchronous Optical Network/SynchronousDigital Hierarchy (SONET) protocol as indicated in a set of standardsprovided by the American National Standards Institute or SynchronousDigital Hierarchy (SDH) as indicated in a set of recommendationsprovided by the International Telecommunications Union. However, theseare merely examples of layer 2 devices that may provide a parallel datasignal for transmission on an optical transmission medium, andembodiments of the present invention are not limited in these respects.

The layer 2 section 108 may also be coupled to any of severalinput/output (I/O) systems (not shown) for communication with otherdevices in a processing platform. Such an I/O system may include, forexample, a multiplexed data bus coupled to a processing system or amulti-port switch fabric. The layer 2 section 108 may also be coupled toa multi-port switch fabric through a packet classifier device. However,these are merely examples of an I/O system which may be coupled to alayer 2 device and embodiments of the present invention are not limitedin these respects.

The layer 2 device 108 may also be coupled to the PMA section 106 by abackplane interface (not shown) over a printed circuit board. Such abackplane interface may comprise devices providing a 10 Gigabit EthernetAttachment Unit Interface (XAUI) as provided in IEEE Std. 802.3ae-2002,clause 47. In other embodiments, such a backplane interface may compriseany one of several versions of the System Packet Interface (SPI) asdefined by the Optical Internetworking Forum (OIF). However, these aremerely examples of a backplane interface to couple a layer 2 device to aPMA section and embodiments of the present invention are not limited inthese respects.

FIG. 4 shows a schematic diagram of a system 200 to transmit data in andreceive data from an optical transmission medium according to anembodiment of the system shown in FIG. 3. An optical transceiver 202comprises a laser device 208 to transmit an optical signal 210 in anoptical transmission medium and a photo detector section 214 to receivean optical signal 212 from the optical transmission medium. The photodetector section 214 may comprise one or more photodiodes (not shown)for converting the received optical signal 212 to one or more electricalsignals to be provided to a TIA/LIA circuit 220. A laser driver circuit222 may provide a current signal 216 to the laser device 208 in responseto a data signal from a PMA section 205. The laser device 208 may thentransmit optical signal 210 in response to the current signal 216.

FIG. 5 shows a schematic diagram of a laser driver circuit 322 accordingto embodiment of the physical medium dependent section shown in FIG. 4.A laser device comprising a laser diode 306 receives a current signalfrom the laser driver circuit 322 and generates a light signal 310 inresponse to the input current. A photodiode 308 may be used to monitorthe output power of the light signal 310 by providing an output currentto a control circuit 302 over a resistor 312. A voltage at the resistormay be indicative of the output power of the laser diode 306.Alternatively, instead of passing the output current over the resistor312, a TIA may be coupled to receive the output current and provide anoutput voltage to the control circuit 302. However, these are merelyexamples of how an output power of a laser device may be measured andembodiments of the present invention are not limited in these respects.

The laser driver circuit 322 comprises a current source circuit 316 togenerate a modulation current component (I_(MOD)) and a current sourcecircuit 314 to generate a bias current component (I_(BIAS)). A switchtransistor pair comprises switch transistors 318 and 320 to modulate aswitched modulation current output in response to a data signal (e.g.,from a PMA section) applied to gates of the switch transistors 318 and320. The switched modulation current and bias current components may beadditively combined using techniques known to those of ordinary skill inthe art of analog circuit design to provide a current signal forpowering the laser diode 306.

In the illustrated embodiment, the current source circuits 314 and 316may adjust the magnitudes of I_(MOD) or I_(BIAS) in response to controlsignals from the control circuit 302 to adjust the output power of thelaser diode 306. In one embodiment, the current source circuits 314 and316 may increase or decrease the magnitudes of I_(MOD) or I_(BIAS)continuously over a time period to enable data recovery circuitry at areceiving end (not shown) to respond to changes in the output power.Alternatively, the current source circuits 314 and 316 may change themagnitudes of I_(MOD) or I_(BIAS) as a step function for a fasterresponse. However, these are merely examples of how a modulation currentor bias current may be adjusted in response to a control signal andembodiments of the present invention are not limited in these respects.

According to an embodiment, the laser driver circuit 322 may beassociated with preset system parameters (e.g., preset by amanufacturer) such as a target reference average output power (P_(REF))and a target modulation power or swing output power (P_(MOD)). FIG. 6shows a flow diagram illustrating a process 400 to adjust I_(BIAS)and/or I_(MOD) according to an embodiment of the laser driver circuit322. At bubble 402, a reset event may be detected such as a power upevent. Block 404 may set internal parameters and factory defaultparameters including, for example, P_(MOD) and P_(REF) . Blocks 406through 414 comprise a processing loop that may then be executed until asubsequent reset event.

Block 406 and diamond 408 may detect an event or condition to initiate achange in I_(BIAS) or I_(MOD) to maintain the average output power ofthe laser diode 306 at about P_(REF). Such an event or condition mayinclude, for example, a change in the temperature of the laser diode 306(e.g., as measured by a thermistor (not shown)) or a change in theaverage output power (P_(AVE)) from the laser device 306 (e.g., asmeasured from the output of the photodiode 308). However, these aremerely examples of a condition or event that may initiate a change inI_(BIAS) or I_(MOD), and embodiments of the present invention are notlimited in these respects. Block 406 measures P_(AVE) and diamond 408determines whether P_(AVE) is within a suitable range (e.g., as definedin parameters set at block 404) about P_(REF). If P_(AVE) is not withinthe suitable range about P_(REF), block 408 may adjust I_(BIAS) untilP_(AVE) is within the suitable range.

According to an embodiment the current source circuit 314 may adjustI_(BIAS) in discrete current increments (ΔI_(BIAS)) in response to adigital control signal from the control circuit 302. At block 410,I_(BIAS) may be adjusted by one or more increments ΔI_(BIAS) (e.g.,added to or subtracted from I_(BIAS)) until P_(AVE) is within a suitablerange. As illustrated in FIG. 7, for example, I_(BIAS) may be adjusteduntil P_(AVE) is within the range P_(REF) +/−nΔP_(REF) where ΔP_(REF)and n define a predetermined tolerance for P_(AVE).

Following the adjustment of I._(BIAS) at block 410, the control circuit302 may approximate a slope efficiency (Eƒ_(slope)) associated with thelaser diode 306 at block 412. According to the embodiment of FIG. 7,Eƒ_(slope) may be approximated based upon a discrete current incrementΔI_(BIAS) and the change in average output power (ΔP_(o)) resulting fromthe last current increment ΔI_(BIAS) added to or subtracted fromI_(BIAS) at block 410 (to place P_(AVE) within the range P_(REF)+/−nΔP_(REF) as follows:Eƒ _(slope) ≈ΔP _(o) /ΔI _(BIAS)

According to an embodiment, the control circuit 302 may provide acontrol signal to the current source circuit 316 to maintain amodulation current I_(MOD). At block 414, the control circuit 302 maydetermine I_(MOD) based upon P_(MOD) and the slope efficiencyapproximation ΔP_(o)/ΔI_(BIAS) as follows:I _(MOD) =P _(MOD)/(ΔP _(O) /ΔI _(BIAS))

To maintain P_(AVE) within a suitable operating range, I_(BIAS) may bereduced by an amount of current based upon the adjusted modulationcurrent I_(MOD). In the presently illustrated embodiment, it may beassumed that the output power of the laser diode 306 is approximatelysymmetric about P_(AVE) in response to the modulation current I_(MOD).Accordingly, the bias current I_(BIAS) determined at block 410 may bereduced by about half of any increase to I_(MOD) to maintain P_(AVE)within a suitable operating range. However, this is merely an example ofhow a bias current may be reduced to maintain the average output powerof a laser diode within a suitable range and embodiments of the presentinvention are not limited in these respects.

FIG. 8 shows a diagram illustrating a laser driver circuit 522 accordingto an embodiment of the laser driver circuit 322 illustrated withreference to FIGS. 5 and 6. An analog to digital convener (ADC) 520 mayprovide digital samples of a voltage signal from a monitor photodiode tocontrol logic 502 used for measuring P_(AVE) and ΔP_(o), according to anembodiment of the control circuit 302 described with reference to FIG.6. Current source circuits 514 and 516 each comprise one or moredigital-to-analog converters (DACs) to provide a current at a magnitudecontrolled by a digital signal from the control logic 502. Circuitry toform such DACs to generate a digitally controlled current may beimplemented using techniques known to those of ordinary skill in the artof analog circuit design.

FIG. 9 shows a flow diagram illustrating a process 600 according to anembodiment of the processing in the block 414 portion of process 400shown in FIG. 6. and the laser driver circuit 522 shown in FIG. 8. A DAC516 may generate I_(MOD) as an integer multiple of ΔI_(MOD) generated asfollows:I _(MOD) =N _(MOD) ×ΔI _(MOD)

In the illustrated embodiment, N_(MOD) may be calculated as an integerfrom the approximated slope efficiency Ef_(slope) (calculated at block412) and diamond 604 may determine whether there has been a change toN_(MOD) resulting from any change to Eƒ_(slope). Diamond 604 may compareN_(MOD) as calculated at block 602 to CurrN_(MOD) which is a previouslystored value of N_(MOD) (e.g., initialized at block 404 followingreset). If N_(MOD) has changed, block 606 may provide an updated N_(MOD)signal as CurrN_(Mod) to DAC 516 to generate I_(MOD) as an integermultiple CurrN_(Mod) of discrete current increments ΔI_(MOD).

Since any changes to I_(MOD) (resulting from changes in N_(MOD)) maycause a change in P_(AVE), I_(BIAS) may be adjusted to maintain P_(AVE)within a suitable range. At block 410, the control logic 502 may providea digital control signal to a DAC 518 to increase or decrease the outputcurrent from DAC 518 by the discrete current increments ΔI_(BIAS) toplace P_(AVE) within a suitable operating range as illustrated withreference to FIG. 7. At block 606, the control logic 502 may providesignal CurrN_(MOD) as a digital control signal to a DAC 516 to generateI_(MOD) based upon the approximated slope efficiency Eƒ_(slope).

According to an embodiment, the DAC 516 may provide I_(MOD) as aninteger multiple (N_(MOD)) of discrete current increments ΔI_(MOD) basedupon Eƒ_(slope).

Accordingly, the DAC 516 may generate a modulation currentI_(MOD)=CurrN_(MOD)×ΔI_(MOD) in response to a digital control signalfrom the control logic 502 to maintain a swing output power at about thetarget modulation power P_(MOD). To maintain the average power P_(AVE)within a suitable operating range, the current source circuit 514 mayoffset the output current from the DAC 518. A DAC 508 may generate acurrent in response to a digital signal ΔN_(MOD) which represents achange in CurrN_(MOD) (updated at block 608). Here, the DAC 508 maygenerate a current ΔN_(MOD)×ΔI_(MOD). Half of this output current fromthe DAC 508 may be subtracted from the output of DAC 518 to provideI_(BIAS) using techniques known to those of ordinary skill in the art ofanalog circuit design.

While there has been illustrated and described what are presentlyconsidered to be example embodiments of the present invention, it willbe understood by those skilled in the art that various othermodifications may be made, and equivalents may be substituted, withoutdeparting from the true scope of the invention. Additionally, manymodifications may be made to adapt a particular situation to theteachings of the present invention without departing from the centralinventive concept described herein. Therefore, it is intended that thepresent invention not be limited to the particular embodimentsdisclosed, but that the invention include all embodiments falling withinthe scope of the appended claims.

1. A method comprising: determining a magnitude of a modulation currentprovided to a laser device based upon an approximated slope efficiencyand a target output power swing, wherein the approximated slopeefficiency is based upon at least one discrete incremental change in abias current to the laser device and at least one change in output powerfrom the laser device, and wherein the target output power swing is apredetermined range of output power values of the laser device; anddetermining a magnitude of the bias current provided to the laser devicebased upon the approximated slope efficiency.
 2. The method of claim 1further comprising: selectively adjusting the bias current based uponthe approximated slope efficiency in response to a change in themodulation current.
 3. The method of claim 2 wherein adjusting the biascurrent includes: maintaining the output power of the laser devicewithin a predetermined power range.
 4. A system comprising: a laserdevice, adapted to be coupled to an optical transmission medium, fortransmitting an optical signal in the optical transmission medium inresponse to a data signal; and a laser driver circuit for providing apower signal to the laser device, the laser driver circuit comprising:logic to determine a magnitude of a modulation current provided to thelaser device based upon an approximated slope efficiency and a targetoutput power swing, wherein the approximated slope efficiency is basedupon at least one discrete incremental change in a bias current to thelaser device and at least one change in output power from the laserdevice, and wherein the target output power swing is a predeterminedrange of output power values of the laser device; and logic toselectively determine a magnitude of the bias current provided to thelaser device based upon the approximated slope efficiency.
 5. The systemof claim 4 further comprising: logic to selectively adjust the biascurrent in response to a change in the modulation current.
 6. The systemof claim 5, wherein the logic to selectively adjust the bias currentincludes: logic to maintain the output power of the laser device withina predetermined power range.
 7. The system of claim 4 furthercomprising: a framer to provide the data signal to the laser device. 8.The system of claim 7 further comprising: a switch fabriccommunicatively coupled to the framer.
 9. The system of claim 4 furthercomprising: an Ethernet MAC to provide the data signal at a mediaindependent interface.
 10. The system of claim 9 further comprising: abus coupled to the Ethernet MAC.
 11. The system of claim 9 furthercomprising: a switch fabric coupled to the Ethernet MAC.
 12. The systemof claim 4, further comprising a layer 2 section logic configured toprovide the data signal to the laser device.
 13. The system of claim 4,further comprising a physical medium attachment (PMA) section logicconfigured to provide the data signal to the laser device.
 14. Thesystem of claim 4, further comprising a physical medium dependent (PMD)section logic configured to provide the data signal to the laser device.